The present invention relates to a system and method for reducing harmonics in a circuit, and in particular, to an autotransformer-based system and method of current harmonics reduction in a circuit.
AC to DC conversion is used in various applications such as motor drives. A three-phase diode bridge is a typical example of an AC to DC converter. These devices draw line currents that are rich in harmonics during the conversion process. With the widespread use of such non-linear devices to satisfy our technological needs, it has become necessary to tackle problems associated with current harmonics such as overheating of distribution transformers, transmission lines, voltage distortion and power system instability, all of which can lead to power system breakdowns. This has become all the more imperative due to the growing presence of loads that need good quality power such as computers.
Several methods have been developed over the years to reduce line current harmonics. One approach specifically geared towards diode bridge type loads is to increase the number of line phases connected to the diode bridge and hence the number of rectification pulses. However the use of bulky line frequency magnetics is a drawback. In addition, increasing the number of pulses to achieve greater harmonic current reduction usually results in complicated transformer construction and an increase in cost and size.
FIG. 1 shows the schematic diagram of a conventional wye-wye-delta transformer based 12-pulse rectification scheme. The delta-connected windings of the transformer produce a new set of three-phase voltages that lead by 30° with respect to the wye-connected winding voltages. Additional line inductors may be inserted in both lines to further reduce line current distortion. In this case, the entire load power flows through the transformer windings resulting in a bulky and costly solution.
The configuration in FIG. 2 uses a wye-delta transformer to produce a second set of three phases that is 30° out of phase with respect to the mains line-voltages. As shown, the mains lines are connected directly to one of the diode bridges. As compared to the transformer of FIG. 1, the wye-delta transformer is rated for only half the load power and hence is smaller in size. However, the diode bridges have to share power equally in order to achieve good harmonic cancellation. For this, inductors have to be inserted in the main lines. This is done to match the leakage impedance of the wye-delta transformer and act as line impedance to the second diode bridge.
In the 12-pulse scheme shown in FIG. 3 and further described in U.S. Pat. No. 6,101,113, an autotransformer is used to generate the second three-phase set. The use of an autotransformer reduces the overall size of the scheme. However the new set of voltages generated through the transformer is not isolated with respect to the original mains inputs. This can cause undesirable interaction between the voltages in either set. Such an interaction takes place through the diode bridge connections and results in the flow of triplen harmonic currents in the system. As a result, true 12-pulse operation is inhibited. In order to prevent interaction between the two voltage sets, a zero-sequence-blocking transformer, which provides a high zero-sequence impedance to triplen harmonic currents, or an autotransformer constructed on a four-limb core must be used. In both cases, this significantly increases the overall size and cost of the scheme. Further, like in the other conventional schemes shown in FIGS. 1 and 2, this scheme requires equal power sharing between the diode bridges for good harmonic current reduction. Hence, matched line impedances are required at both diode bridge inputs.
Not only is the reduction of line harmonics desirable, current standards for industrial and residential power electronic equipment such as IEC-555 and EN-61000 require that these be within limits. These standards are now being widely followed in Europe. In the US, IEEE-519 recommendations setting limits on harmonic current generation and source voltage distortion by power electronic equipment are being followed on a voluntary basis.
The present invention overcomes the drawbacks present in existing schemes. In particular, the invention uses interaction between the auxiliary voltages generated by the autotransformer and the main voltages to generate additional three phase voltage sets suitable for increased pulse rectification operation, thus obviating the need for special transformer construction, additional magnetics like zero-sequence-blocking devices, and equal power sharing between diode bridges. Since the autotransformer of the present invention is used exclusively for harmonic current reduction and not for load power delivery, it has a power rating significantly smaller than the transformers used in existing rectification schemes. This in turn results in an extremely simple autotransformer configuration that is compact, cost-effective and rugged and which provides an ideal retrofit solution that fully utilizes the rating of the diode bridge present in an existing load device connected thereto. The present invention also meets the performance objectives specified in IEEE 519 recommendations for sources with impedance 2% or less.